In general, a bearing cage is manufactured by injection molding. Specifically, as shown in FIG. 11, a bearing cage is manufactured by forming a circular cavity 140 corresponding to a bearing cage which is a molded body in a mold, injecting a melted resin material (thermoplastic resin) from a resin injection gate 150 provided on a peripheral portion of the cavity 140, and cooling and solidifying.
The melted resin injected to the cavity 140 forms two flows and flows to two sides in a circumferential direction in the cavity 140, then merges at a position on an opposite side facing the resin injection gate 150 in a radial direction, and is joined to each other, thus a weld line 100W is formed. In general, it is well-known that since such an injection molded resin cage for a bearing is a cage in which the melted resin is fused and integrated, uniform mixing of the melted resin does not occur, and strength in the weld line 100W degrades.
In a case where reinforcing fiber materials such as glass fiber, carbon fiber, and metal fiber were added as reinforcement materials to the melted resin, since the reinforcing fiber materials are oriented perpendicular to a flow direction of the melted resin in the weld line 100W, a reinforcing effect does not appear. Further, in a part other than the weld line 100W, since the reinforcing fiber materials are oriented parallel to the flow direction of the melted resin, a difference in strength between the part and the weld line becomes large.
In this ways, the resin cage for a bearing manufactured by injection molding often breaks due to a weld line whose strength is weak. In particular, when the weld line is formed at a portion where stress is most liable to concentrate (for example, a bottom portion whose axial thickness is thinnest in a pocket or a curved portion at a corner where an annular portion intersects with a pillar), damage is liable to occur at this portion, and durability of the cage is impaired. Therefore, the following measures have been taken conventionally.
In a method for manufacturing a synthetic resin cage of Patent Document 1, gates are provided in a plurality of places in a circumferential direction of a cavity of a mold. Among a plurality of regions between the gates, a circumferential distance of some regions is longer than a circumferential distance of the other regions. Further, in a region where the circumferential distance is long, a resin reservoir is provided in a merging place of the injected resin material. As a result, the merging injected resin material flows from the cavity into the resin reservoir, so as to prevent degradation of the weld line strength.
In a resin cage of Patent Document 2, the total number of pockets is set to an odd number, and the number of pockets arranged between gates is set to a most equalized number. The resin reservoir is located in any one of the pillars on both sides of the pocket located at a circumferential center between the gates in which the pocket number is odd. As a result, a weld line formed in a region between the gates in which the pocket number is odd is formed in a position distant from a bottom portion of the pocket in a circumferential direction so as to improve rigidity of the cage.
In a method for manufacturing a resin cage for a bearing of Patent Document 3, at least one first resin reserving portion where a melted resin flows in before a weld line portion is formed in a cavity and at least one second resin reserving portion where a melted resin flows in after a weld line portion was formed in a cavity are provided at a peripheral portion of the cavity. Thus, by properly setting a position where the first resin reserving portion is provided, an occurrence position of the weld line portion is controlled so as to suppress formation of the weld line portion in a part where sufficient strength of the resin cage for a bearing is required. In addition, orientation of reinforcing fibers in the weld line portion is disturbed by the second resin reserving portion where the melted resin flows in after the weld line portion was formed so as to improve the strength of the weld line portion.